Fundamentals of Air Cooling
Everybody knows about the benefits of a cool breeze on a sunny day, a principle that is scientifically supported by the second law of thermodynamics and practically exploited in a number of air cooling systems to cool many different applications. In this article, we would like to share some basic ideas and simplified formulas on air cooling, and briefly discuss alternative cooling strategies, especially in the context of data centers and crypto mining.
For each medium, whether it be a gas or a fluid, a property called “specific heat capacity” determines the amount of energy that can be accepted by a specific amount of the medium. In the case of air at about 20 °C, this constant equals ~1.01 J/g*K. This means that 1 cubic meter of air, which weighs ~1,2 Kg at sea level pressure, can accept 12 KJ of thermal energy from a heat source with a temperature change of 10° C. Since KJ/s are the same as KW, a heat source of 12 KW power will be able to heat up a volume of ~1 cbm air from 20° C to 30 ° C in one second. This also means that cooling of a 12 KW heat source requires an airflow of ~ 1 cbm/s at a delta t of 10 °C.
These relations result in the simple equation of
with V being the required airflow in cbm/s, Q being the thermal heat in kJ/s or KW, Cp being the above introduced specific thermal capacity of the air, ρ being the density of the air and delta t being the difference between inflowing cooling air and exhaust air. This equation states that the volume flow is directly proportional to the applied heat and inversely proportional to the delta t in the denominator. Doubling the delta T will halve the airflow required for cooling, and doubling the thermal load will double the airflow required for cooling.
Other practical examples demonstrating the ease of virtually fail-safe air cooling are air-cooled airplane engines or the highly reliable air-cooled engines that have been used for decades — most successfully in Volkswagen or Porsche cars.
Advantages / Disadvantages of air cooling
Advantages:
Extremely simple structure (motor and fan blades)
Extremely maintenance-friendly by use of maintenance free ball bearings and brushless motors (maintenance free system)
Extremely fail-safe
Easily scalable (within limits)
Extremely energy efficient (fraction of power required by traditional A/C cooling solutions)Disadvantages:
Delta-T directly proportional to cooling efficiency, means high airflows required at high ambient air temperatures / small delta t.
Requires non-trivial aerodynamic considerations
Requires application-specific designs
Traditional refrigeration cycle air conditioning
Traditional air conditioning (A/C) is likely the most widely used strategy to remove thermal energy from the inside of buildings, cars and refrigerators. Even though it is widely used, this technology makes use of a set of error-prone components that require a significant amount of energy themselves.
How does traditional A/C work, and why is it not suitable for successful, efficient remote-managed data centers?
In brief, a specific coolant is compressed and condensed outside of the building, pumped through a hot condensation coil, then through hoses to the inside, and finally is evaporated inside the building that is to be cooled. Air conditioning systems used in IT data centers are similar to those A/C units used in households, but often operate at a higher level of precision and redundancy to minimize thermal downtimes of the data centers.
Although a standard A/C system may seem quite simple at first, it is always comprised of a series of components that rely on each other: 1. An evaporation coil, 2. a condensation coil, 3. fans at the coils and 4. a heat-producing and vulnerable compressor. Failure of a single component, including small leaks in the coolant-containing system, may result in a total failure and fatal breakdown of the whole system in consequence. One key advantage of traditional A/C systems, however, is the capability to generate temperatures below the ambient (outside) air temperatures. This may be considered the key competitive advantage of a traditional A/C system over all other cooling solutions.
Energy-wise, however, the removal of thermal heat by A/C requires an extraordinarily large amount of energy that roughly, as a rule of thumb, adds up to the amount of energy that is to be removed from the inside of the building. In case of an IT data center located either inside of a closed building or a shipping container, this implies precise power supply planning and making sure that requirements for about twice the power that is required by the IT hardware are met. Even modern A/C units require about 30 KW of power for removing the heat generated by a 50 KW server array (citation Schneider paper on IT-A/C). This demonstrates that traditional A/C cooling has no place in efficient, economic, competitive crypto mining endeavours.
Envion’s thoughts on traditional A/C in crypto mining
Although A/C has some key advantages over other types of IT infrastructure cooling, it has no future in successful crypto mining endeavours. Key advantages are cooling of target structures below ambient air temperature and A/C could be deployed with marginal design efforts to any given infrastructure.
The main disadvantages of A/C are that it always requires extremely high amounts of energy and is really prone to failure. Chained components result in fatal system failures upon the failing of a single component. Additionally, A/C systems require regular maintenance. Last but not least, the cold airstream from an improperly installed A/C often results in the critical cooling of metal structures below the dew-point, resulting in local condensation/humidity even inside of IT hardware.
Immersion cooling with water, mineral oils or halogenated hydrocarbons
Another option to directly cool IT equipment is to fully immerse the hardware in a fluid with a higher specific heat capacity than air. Such strategies seem to be discussed and even commercially offered more frequently with the recent rise of cryptomining activity. There are strategies ranging from immersing the precious equipment into pure water, but also into mineral oil. The fluids are then circulated through specific heat exchangers. One advantage of such systems is that under certain circumstances, the density of the IT components may be increased compared to air-cooled solutions. This may be relevant whenever space is a key issue. This, in our opinion, is hardly ever the case. In contrast, it is much more likely that the local capacity is the limiting factor. Moreover, the heat exchanges often require an additional, complicated infrastructure that is dependent on e.g. cooling water (from a river) or other, separate cooling systems such as cooling towers. After all, such immersion cooling systems require special, or at least highly modified, hardware (no fans etc).
Envion’s thoughts on immersion cooling:
In summary, we at envion strongly believe that finding a little cooling river right next to the mining sites, troubleshooting or even reselling oil immersed and fan-amputated hardware will pose greater issues than space will. In our opinion, immersion cooling has no future in cryptomining.
Envion’s proprietary cooling concept: As simple and efficient as possible!
When envion’s mobile mining units (MMUs) were designed, our first and foremost goal was to keep the system as simple as possible, introducing as little maintenance-prone parts as possible, and to keep it as energy efficient as possible.
Because of these principles, using a standard A/C system has never been an option for envion. Instead, an optimized, patent-pending strategy of air cooling was the way to go. And envion had to think of a way to most efficiently direct the cooling air to the targets inside of the MMUs. The underlying aerodynamic design principles are part of a series of more than 30 claims that that envion has submitted for patent application and will, for obvious reasons, not be covered in full detail in this article.
As stated in the introduction, the airflow required to remove a certain amount of energy under ideal circumstances can easily be calculated by the following formula:
For the design of the various facets of envion’s cooling system, we first had to experimentally determine some mining-related constants. One of the most important and most limiting constants was certainly the maximum allowable ambient feed-in air temperature at which the mining equipment was still performant. This temperature was slightly different for ASIC and GPU miners, and slightly in favor for ASIC miners. At the level of the MMU, this maximum allowable operating temperature determines the maximum temperature inside the MMU, which may also be called the maximum “exhaust temperature”. This temperature represents the upper side of the delta t term in the above equation. At a given maximum airflow that is determined by the fans, the minimum delta T required to remove a total thermal energy (Q) can be calculated. Or the maximum thermal energy release inside the MMU can be assessed for a given delta t and airflow. Of course, these considerations always assume a close-to-ideal airflow / air exchange rate.
Within the MMU
To achieve an even air exchange within the MMU and to avoid trapping air in certain spaces, an even distribution of ventilation holes, fans and ducts is an essential prerequisite. Simply installing a fan to one side of the container and inserting a hole (with or without a filter) at the other side will certainly result in a circulation between the fan and the hole. However, the resulting airflow will not be close to an “air exchange rate” of the compartment. Instead, the air will mainly flow from one hole to the other, leaving large areas of the container insufficiently ventilated. To overcome this problem, envion has created a system of holes, ducts and air guides, guiding fresh, individually filtered outside air directly towards the (evenly distributed) mining hardware. The system of ducts and holes and the strategic placement of fans ensures an even, efficient and thorough air exchange within the MMU. Flow simulations in the field have proven its efficiency.
Fan size: All that matters is diameter
envion’s concept makes use of the basic aerodynamic principle that a propeller, be it an impeller of an extraction fan, or a propeller of an airplane, increases in efficiency with an increasing diameter (so.-called aspect ratio). There are also differences in efficiency within a class of fans at an equal diameter due to a different form factor, but a larger diameter has, by far, the most significant impact on fan efficiency. This principle can also be observed in aviation, with the Swiss long-range experimental solar-powered aircraft Solar Impulse’s propellers being one of the most recent examples of high efficiency propellers.
Envion also makes use of this principle by using large diameter industrial standard fans that can transport a vast amount of air using relatively small amounts of energy. One additional advantage of using a large diameter fan can be found in the relation between airflow and power, where airflow is a cubic function of the power. According to this relation, a reduction of the airflow from 100% peak flow to 50% peak flow may result in a power reduction by 75%. A precise regulation of the fan speed can, therefore, result in significant energy savings by increasing the efficiency of the total system.
Figure 1 demonstrates the relation between speed, diameter, form factor and the impact on total efficiency.
In a second step, scenarios for envion’s key use cases were thoroughly assessed (through a greenhouse, industrial hall, open air, hot climate, dusty environment). Cooling system components were selected and experimentally confirmed for different environments.
Figure 2 represents a sketch of the system curves of the container using coarse, intermediate and high-density filters. The ordinate represents the pressure in Pascals and the abscissa represents the airflow in metric units (cubic meters per hour). These curves basically state the required pressure to realize a corresponding airflow. The higher the resistances of the filters, the higher the pressure difference between the inside and the outside of the container to realize a specific airflow rate.
The dashed lines represent the characteristic fan curves, indicating the airflow at a specific pressure. Curve №3, representative of two efficient standard fans with a diameter of about 400 mm, indicates a maximum pressure of about 170 Pa at about 6000 cbm/h, and a maximum free-blowing airflow of slightly above 12000 cbm/h. Using this type of fan in the container with coarse filters results in operating point 1, which is the intersection between the system curve and the fan curve. Fan Curve 2 represents the characteristic curve of a larger diameter fan set of 630 mm diameter. These fans can operate at even higher pressures and realize a free blowing airflow of almost 15,000 cbm/h. Using these fans in combination with intermediate filters results in Operating Point 2 at about 140 Pa and a maximum airflow of 13,500 cbm/h.
For extreme conditions including high ambient air temperatures, high levels of pollution, dust and/or reserve cooling capacity or higher IT hardware density, ultra high-performance fans can be deployed and are currently under investigation at envion’s lab. The characteristic curve of two of these fans is depicted by the dashed line №1. Most importantly, these high power fans are of only 400 mm diameter. This results in a less-than-optimal efficiency, but plenty of cooling power at times where it may be needed. The operating point even for the fine filters is at a maximum airflow of 14,500 cbm/h with only two fans.
According to the above introduced equation
an airflow of 12,000 cbm/h corresponds to an airflow of 3,33 cbm/s. This results in a maximum thermal heat dissipation rate of 4 kJ/s per temperature difference of 1 K (or 1 °C). A delta T of 10 °C would, therefore, at the airflow rate of 3,33 cbm/s, be sufficient to eliminate the thermal energy corresponding to a 40 KW heat source under optimal aerodynamic conditions. At envion, our very first laboratory testing of our aerodynamic calculations using heat sources of around 55 KW resulted in a delta T of a mere 15 °C at about 12,000 cbm/h. This was only slightly above theoretically predicted delta T during steady state operations. Determinations of the maximum allowable ambient air temperatures before thermal shutdown of GPU rigs and ASICS occurred resulted in values well above 45° C. Using three of our ultra high-power fans in the coarse filter setup, the maximum airflow rate is predicted at about 22,000 cbm/h. This will allow us to run a 55 KW MMU at maximum outside air temperatures of > 40 °C at full hashing power. Depending on the deployment site, even higher temperatures can be handled by scaling up the cooling power, however, a temporary decrease in hashing power is currently considered the better alternative.
With respect to standard ASIC miners, envion has also submitted a patent application for the direct, passive feed-in of fresh outside air into the ASIC miners, and guidance of exhaust air directly towards the large diameter fans. This results in most efficient cooling of the ASIC machines and makes internal fans obsolete. A failure of an internal fan is, therefore, no issue for envion. In the final series of ASIC-hosting MMUs, a scenario with ASIC miners completely lacking small diameter fans is currently being investigated.
Under most operating conditions, the power of a single fan is sufficient to cool the equipment, so that failure of a fan will be noticed, but will not compromise mining power at all. The use of virtually maintenance-free fans further facilitates the whole cooling process, and, in the unlikely event of a fan failure, our fan units can be replaced within minutes. The use of high efficiency three-phase 400 V electrical motors allows us to use sophisticated sine wave frequency converters and microcontroller-governed reversal of the fans. In combination with stepper-motor controlled fan gates (currently still in manual mode), a filter cleaning procedure can be run by simply having fans run in reverse at full throttle for a short period of time. This cleans the filters from large debris such as leaves, plastic bags or other large to medium-sized particles that are stuck to the filter membranes.
